Set the docker graph path instead of a volume

DockerInDocker requires you mount the docker graph into a volume, since
AUFS *inside* AUFS is tricky business. Instead of making use of some
magical environment varaible and mounting a volume, we can now configure
the graph path to a directory inside of the sandbox (jpetazzo/dind#39)
Merge branch 'master' into fix/remove-var-lib-docker

Conflicts:
	portainer/app/scheduler.py
Merge pull request #15 from duedil-ltd/fix/remove-var-lib-docker

Set the docker graph path instead of a volume